The present invention relates to cutting heads and, more particularly, to nozzle assemblies for laser cutting heads which employ capacitance or like non-mechanical measurement to control the spacing of the nozzle from the surface of the workpiece.
For cutting workpieces, laser cutting machines have a laser cutting head with a nozzle at the end adjacent the workpiece. The laser cutting head includes a lens system which focuses the laser beam through an aperture in the nozzle. In order to obtain uniform cutting action, it is necessary for the focal point of the laser beam to be at an optimum distance from the surface of the workpiece. If the workpiece to be cut is not perfectly flat, it is necessary for the focal point of the beam and generally the entire laser cutting head, to be moved upwardly and downwardly to maintain the desired spacing of the focal point relative to the surface of the workpiece.
To effect precision movement of the head in this manner, it is necessary to have a system for measuring the distance between the surface of the workpiece and the nozzle. The distance between the workpiece and nozzle is readily determined by measuring variations in capacitance without mechanical contact. To be able to measure the capacitance, an electrically conductive nozzle must be supported in a non-conductive receptacle in the cutting head, and the electrically conductive nozzle must be connected by a conductor to the measuring instrument.
Because the nozzle is a part which must be periodically replaced, some prior devices have designed the nozzle and receptacle with complementary screw threaded surfaces. One nozzle assembly employing this type of construction has had a receptacle with an inner or female screw thread in which is engaged the outer or male screw thread of the nozzle. A conductive path is formed by a metallized coating on the screw thread of the receptacle, and it is connected by a wire in the receptacle to a conductive contact point on the side opposite the nozzle.
The disadvantage of this prior art structure is that the screw thread in the receptacle, which is usually of ceramic material, is relatively difficult to produce, and the metallizing of the threads is expensive and imprecise. Further, this screw thread is often very brittle and there is a problem in that the metal in the screw threads wears out and tears. This results in a poor electrical connection, or even a break in electrical connection, between the nozzle and the receptacle, thereby causing the measuring instrument to become no longer functional.
Moreover, it is difficult to manufacture a receptacle of insulating material with screw threads to close tolerances. To maintain good concentricity, surface finishes, and critical measurements, surfaces must be ground after the ceramic is fired. Grinding the ceramic is a costly process, and could prove to result in poor quality of operation in the laser cutting head.
Another previous device is disclosed in U.S. Pat. No. 5,128,508 to Hans Klingel, hereby incorporated by reference. The Klingel device attempts to solve the problems of the previous devices by including a conductive insert having a body portion of generally annular cross section with a central passage extending therethrough, wherein the wall of the passage is internally threaded. The threads of the conductive insert mate with complementary threads on a neck portion of a conductive nozzle. The conductive insert is adhesively bonded to an annular receptacle of insulating material. A conductive pin is soldered to the insert, and extends from the insert, through the receptacle to provide a conductive path from the nozzle to the cutting head.
However, the Klingel device suffers many disadvantages. The solder connecting the conductive pin to the conductive insert looses its hold under conditions of intense heat that are typical in such a device. Further, the glue connecting the conductive insert to the receptacle of insulating material breaks down, and looses its hold, after extended normal operating use, or after a relatively short use under high heat applications. Both a failure in the solder connection and a failure in the adhesive connection cause the conductive insert to come loose, thus causing the nozzle, which is connected to the conductive insert, to be out of center or fall out completely. Once the nozzle is out of center, the system will not function properly, and other damage could result. Needless to say, if the nozzle falls out completely, especially during operation of the laser, extensive damage could occur.